The present invention relates to a ceramic heater employed in a glow plug or the like for preheating a Diesel engine and having an ion detecting electrode, and a method for manufacturing the ceramic heater, and a glow plug using the aforementioned ceramic heater and an ion current detecting device using said glow plug.
From the view point of environmental protection of recent years, not only a gasoline engine but also a Diesel engine is desired for reducing the exhaust gas discharged from the engine and the noxious substance in the exhaust smoke. Especially the Diesel exhaust particle (DEP) to be discharged as a main cause of an incomplete combustion in the engine is being recently regulated in Japan. In order to satisfy such desire, moreover, there have been made various proposals on the construction of the engine, the improvement in the combustion control, the exhaust gas treatment using a catalyzer and the improvement in the fuel or lubricant.
In some engine combustion control system of recent years, moreover, there is mounted a mechanism for detecting the engine combustion state as control information. The specific parameters to be measured are exemplified by the internal cylinder pressure, the combustion light or the ion current. Especially the detection of the engine combustion state in terms of the ion current is accepted useful because the chemical reaction situation accompanying the combustion can be directly grasped. Thus, there have been proposed a variety of ion current detecting methods. On the gasoline engine, there has been adopted a detection method, in which the ignition interval is utilized to use the spark discharge gap of the spark plug in the ion current generating portion. However, this method cannot be adopted in the Diesel engine because this engine does not employ the spark plug, as known in the art.
On the Diesel engine, on the other hand, there is mounted a glow plug for warming up the engine. Therefore, the ion current detecting method utilizing the glow plug has been disclosed, for example, in (unexamined) Japanese Patent Kokai Publication Nos. JP-10-89223A, 10-89686A, 10-89687A and 10-122114A. The summary of the principle will be described in the following.
Specifically, the glow plug is provided with a resistance heating heater arranged in the combustion chamber. This heater is continuously fed with an electric power to heat till the warm-up of the engine is completed, but is not basically used after the warm-up was ended. Therefore, the glow plug is used as an ion current detecting probe. In order to add the ion detecting function to the glow plug, more specifically, an additional structure is made such that an ion current detecting electrode portion is so mounted on the resistance heating element of the heater that a portion of the electrode surface is exposed to the heater surface. At the time of starting the engine, moreover, the warm-up is performed by connecting the resistance heating element with the heating power source to energize it for the heating action. After the end of the warm-up, on the other hand, the power source and the conduction passage are switched for the ion current so that the ion current may be produced between the inner face of the combustion chamber in the grounded engine block and the ion current detecting electrode portion. In case a waveform reflecting the situation of an incomplete combustion is detected in a signal of the ion current, for example, the connection can be switched again to the heating power source to cause the resistance heating element to heat thereby to assist the combustion.
For example, the glow plug disclosed in Japanese Patent Kokai Publication No. JP-10-89686A employs a ceramic heater in which a resistance heating element made of ceramic is buried in an insulating ceramic substrate. As the materials for the resistance heating element and the ion current detecting electrode portion, there are enumerated conductive inorganic compounds such as molybdenum disilicate (MoSi2), pentamolybdenum trisilicate (Mo5Si3), molybdenum silicon carbide (MoxSi3Cy), molybdenum boride (MOB), tungsten carbide (WC) and TiN, etc.
However, the conductive inorganic compound of the above-specified material is relatively satisfactory in the electric characteristics when employed as the resistance heating element, but has the following problems as a material for the ion current detecting electrode portion to contact directly with a hot combustion gas. Specifically, Mo or W or a cation component of those inorganic conductive compounds is defective in that it is easily oxidized in contact with the hot combustion gas, and in that an oxide such as MoO3 or WO3 produced is, because of the trivalence, so volatile that it is seriously exhausted at a high temperature to significantly shorten the lifetime of the ion current detecting electrode portion. Here, Japanese Patent Kokai Publication No. JP-10-89686A has also disclosed a mode in which the exposed surface portion of the ion current detecting electrode portion is coated with a precious metal such as Pt, Ir, Rh, Ru or Pd. However, the precious metal is expensive and is complex in the manufacture steps so that it is not economical. Moreover, the contact with the conductive inorganic compound making the substrate for the coating and the separation or cracking of the precious metal coating portion due to the difference in the linear expansion coefficient are liable to raise problems so that the coating is not preferred from the view point of durability.
It is an object of the present invention, according to one aspect, is to provide a ceramic heater which is better in the durability of an ion current detecting electrode portion and which can be manufactured at a low cost.
It is another object, according to another aspect of the invention, to provide a method for manufacturing the ceramic heater. It is a further object, according to a further aspect of the invention to provide a glow plug and an ion current detecting device employing the ceramic heater, respectively.
Further objects and aspects of the invention will become apparent in the entire disclosure, claims and drawings.
According to a first aspect of the present invention, there is provided a first construction of a ceramic heater comprising:
an insulating ceramic substrate;
a resistance heating element made mainly of conductive ceramic and buried in the insulating ceramic substrate; and
an ion current detecting electrode portion made mainly of conductive ceramic and integral with the resistance heating element in the insulating ceramic substrate and having a portion of its own surface exposed as an ion current detecting face to the surface of the insulating ceramic substrate,
characterized in that the ion current detecting electrode portion is constructed such that a portion including at least a portion of the ion current detecting face is made of a nonmetallic conductive ceramic having a cation component of at least one nonmetallic element.
According to the aforementioned construction of the ceramic heater, the ion current detecting electrode portion is constructed such that a portion including at least a portion of the ion current detecting face is made of a nonmetallic conductive ceramic having a cation component of a nonmetallic element or elements. The nonmetallic conductive ceramic is superior in oxidation resistance to the metallic conductive ceramic which is generally employed as a material for a ceramic resistance heating element and which has a cation component made of a metallic element(s), and also hardly generates high-temperature volatile oxides. By adopting the nonmetallic conductive ceramic as a material for constructing the ion current detecting face, therefore, it is possible to elongate the lifetime of the ion current detecting electrode portion.
In the aforementioned first construction, the nonmetallic conductive ceramic can be made mainly of one kind or two kinds or more of silicide, carbide, nitride or boride of nonmetallic cation element. As the nonmetallic cation element, there can be adopted metalloid such as silicon (Si), germanium (Ge) or selenium (Se), for example. Such one of the aforementioned silicide, carbide, nitride and boride as has an electric conductivity proper for the ion current detection at the working temperature can be properly employed in the present invention.
Of the aforementioned compounds, moreover, one containing silicon carbide as its main component can be properly in the present invention. This compound has a sufficient oxidation resistance even in the working atmosphere in which the temperature rise up to 1,000 to 1,350xc2x0 C. is anticipated in contact with a hot combustion gas, and is far less expensive than the precious metal or the like. Moreover, the produced oxide is little volatile silicon dioxide so that the oxidation exhaustion hardly occurs. Therefore, it is possible to rationally realize a ceramic heater which is more excellent in the durability of the ion current detecting electrode portion and which can be manufactured at a low cost.
The ion current detecting electrode portion may be made of the aforementioned nonmetallic conductive ceramic only at the surface layer portion including the ion current detecting face. In case the nonmetallic conductive ceramic having an especially excellent electric conductivity such as silicon carbide is employed, however, the ion current detecting electrode portion can be wholly constructed of the nonmetallic conductive ceramic. Moreover, the entirety including not only the ion current detecting electrode portion but also the resistance heating element can also be constructed of the nonmetallic conductive ceramic.
It is remarkably effective in the view point for protecting the ion current detecting electrode portion against the oxidation exhaustion to construct the portion including at least a portion of the ion current detecting face, of the aforementioned nonmetallic conductive ceramic. In order to improve the heater temperature rising characteristics better, on the other hand, a material different from the nonmetallic conductive ceramic, that is, a metallic conductive ceramic having a cation component made of a metallic element can be adopted for the resistance heating element. Specifically, the resistance heating element is constructed mainly of the first conductive ceramic phase having the cation component made of the metallic element(s), and the ion current detecting electrode portion is constructed of the second conductive ceramic phase made of the nonmetallic conductive ceramic, as constructed mainly of the aforementioned silicon carbide.
Here in this Specification, in case a terminology xe2x80x9cmain componentxe2x80x9d (or xe2x80x9cas major componentxe2x80x9d or xe2x80x9cmainlyxe2x80x9d) is used on the contained component in a substance being noted, it means the component of the highest weight content in that substance. Moreover, the phrase xe2x80x9ctwo kinds or more components are used as the main componentxe2x80x9d means that the total of the components has a higher weight content than that of any of the remaining individual components. Here on the components of the substance having a structure of a plurality of phases, the main component on the individual constructing elements or constructing compounds can be specified by the aforementioned definitions by deeming the individual phases as the individual substances. On the entire structure, moreover, the xe2x80x9cphasexe2x80x9d to become the major component in the structure can be specified by the aforementioned definitions by deeming the individual constructing phases as the individual components. In the present invention, moreover, the individual substances, which are conceptionally specified by using the terminology of the xe2x80x9cmain componentxe2x80x9d, the xe2x80x9cmajor componentxe2x80x9d and the xe2x80x9cmainlyxe2x80x9d, may contain any kind of by-component so long as the basic actions and effects of the present invention can be achieved.
Moreover, a second construction of a ceramic heater according to a second aspect of the present invention comprises:
an insulating ceramic substrate;
a resistance heating element made mainly of conductive ceramic and buried in the insulating ceramic substrate; and
an ion current detecting electrode portion made mainly of conductive ceramic and integral with the resistance heating element in the insulating ceramic substrate and having a portion of its own surface exposed as an ion current detecting face to the surface of the insulating ceramic substrate,
characterized in that the resistance heating element is made mainly of a first conductive ceramic phase; and in that the ion current detecting electrode portion is constructed such that a portion including at least a portion of the ion current detecting face is made of a second conductive ceramic phase having a better oxidation resistance than that of the first conductive ceramic phase.
In the aforementioned construction, the ion current detecting electrode portion demanded for the oxidation resistance and the exhaustion resistance at a high temperature is constructed such that a portion including at least a portion of the ion current detecting face is made of a second conductive ceramic phase having a better oxidation resistance than that of the first conductive ceramic phase constructing the resistance heating element mainly. As a result, the durability of the ion current detecting electrode portion can be enhanced without sacrificing the performances or the like of the resistance heating element. For example, the first conductive ceramic phase is constructed of ceramic having better electric characteristics demanded as a resistance heating element than those of the second conductive ceramic phase, such as the conductive ceramic phase which has a high electric conductivity at the heater working temperature or which has a low resistance at the beginning of conduction and an excellent temperature rising performance, for example. Then, it is possible to realize an ideal ceramic heater which has both the excellent heater characteristics and the durability of the ion current detecting electrode portion.
In any of the first and second constructions of the ceramic heater of the present invention, the first conductive ceramic phase constructing the resistance heating element mainly may properly employ as a main component one kind or two kinds or more of molybdenum disilicate (MoSi2), tungsten carbide (WC), tungsten disilicate (WSi2), pentamolybdenum trisilicate (Mo5Si3) molybdenum silicon carbide (MoxSi3Cy: 5 greater than xxe2x89xa74, 0 less than yxe2x89xa61, x+y=5), because these are excellent in an electric conductivity at the heater working temperature (e.g., 1,100 to 1,350xc2x0 C.) and in quick temperature rising performance. It is desired that the resistance heating element has a content of the first conductive ceramic phase of 50 to 75 mass %. There may occur a case where the aforementioned effects are unable to be sufficiently achieved, if the content is less 50 mass %, and the intergranular phase based on the sintering agent(s) may be insufficiently formed to fail to form a dense resistance heating element if the content is more than 75 mass %.
In the first construction in which the use of the nonmetallic conductive ceramic is essential, on the other hand, the second conductive ceramic phase constructed mainly of silicon carbide can be properly employed in the present invention. In the second construction, on the other hand, the second conductive ceramic phase should not be limited especially to the nonmetallic conductive ceramic, if it is superior in the oxidation resistance to the first conductive ceramic phase, but can be constructed of not only the aforementioned silicon carbide but also one kind or two kinds or more of titanium nitride, zirconium nitride, hafnium nitride, titanium boride, zirconium boride and hafnium boride, as its major component. From the view point of retaining excellent electric conductivity and oxidation resistance, however, silicon carbide can also be most properly used in the present invention.
In order to improve the lifetime of the ion current detecting electrode portion better, it is possible to construct the structure of the surface layer portion of the ion current detecting electrode portion mainly of the second conductive ceramic phase, such that the remainder excepting the grain boundary binding phase can be constructed of the second conductive ceramic phase. In order to retain the electric conductivity better, on the other hand, the ion current detecting electrode portion can also be constructed of the composite conductive ceramic in which the first conductive ceramic phase and the second conductive ceramic phase coexist. In this construction, a portion of the second conductive ceramic phase should be exposed to the ion current detecting face.
According to a further aspect of the invention, the ceramic heater of the present invention thus far described can be rationally manufactured by the following manufacturing method. Specifically, the method is characterized by comprising: preparing a composite shaped body, in which an electrode shaped portion for the ion current detecting electrode portion and a heating element shaped portion for the resistance heating element are buried in a substrate shaped portion for the insulating ceramic substrate; and sintering the composite shaped body. For example, the following method can be adopted, especially in case the portion of the ion current detecting electrode portion containing at least a portion of the ion current detecting face is constructed of the aforementioned second conductive ceramic phase whereas the resistance heating element is constructed of the aforementioned first conductive ceramic phase. The method comprises: forming a portion of the electrode shaped portion for the ion current detecting face, into a second shaped body containing a material for at least the second conductive ceramic phase; forming an integrated shaped body in which the second shaped body and a first shaped body made mainly of a material for the first conductive ceramic phase and including a portion for the heating element shaped portion are integrated; and burying the integrated shaped body in the substrate shaped portion for the insulating ceramic substrate, to form the composite shaped body. In this case, the integrated shaped body is efficiently formed by an insert molding method, by which the second shaped body is arranged as an insert in a mold so that a compound containing a material for the first shaped body may be injected into the mold.
Next, the glow plug according to a further aspect, of the present invention is characterized by comprising: a ceramic heater as described in the present invention; and a housing having a mounting portion formed for holding the ceramic heater and for mounting the ceramic heater in an internal combustion engine so that the ion current detecting face may be positioned in a combustion chamber. Moreover, the ion current detecting device is characterized by comprising: the aforementioned glow plug of the present invention; a heating power source unit for energizing the resistance heating element of the glow plug to heat; an ion generating power source unit for applying an ion generating voltage to the ion current detecting electrode portion through the resistance heating element of the glow plug; a power switching portion for switching to connect one of the heating power source unit and the ion generating power source unit selectively with the glow plug; and an ion current detecting portion for detecting an ion current to flow to the ion current detecting electrode portion.
According to the aforementioned constructions of the glow plug and the ion current detecting device, the adoption of the ceramic heater of the present invention makes it hard to exhaust the ion current detecting electrode portion and to deteriorate its characteristics and possible to detect the ion current highly accurately for a long time. Therefore, the constructions highly contribute to a reduction in the toxious substance (especially, Diesel exhaust particle) in the exhaust gas or exhaust smoke discharged from the Diesel engine. Moreover, the entirety can be inexpensively constructed so that the ion current detecting device contributing to the environmental protection can spread widely.